Migmatite is a rock formed under high-grade metamorphism by partial melting (anatexis), producing a mixture of metamorphic and newly formed igneous material. It typically occurs in the deeper parts of the continental crust, commonly within Precambrian cratonic blocks, and represents a transitional stage between metamorphic and igneous rocks. Migmatites form during prograde metamorphism when rocks reach temperatures and pressures sufficient for partial melting. The newly formed material, known as the neosome, commonly consists of light-coloured leucosome and darker melanosome, while remnants of the original rock form the paleosome; an intermediate mesosome may also be present. Migmatites commonly display distinctive banding and folded structures, including ptygmatic folds, produced by melt migration and ductile deformation. They have long been regarded as linking regional metamorphism, crustal melting, and the origin of granitic rocks, and remain central to research on the evolution of the continental crust.
The diagenesis - metamorphism sequence
Migmatite is the penultimate member of a sequence of lithology transformations first identified by Lyell, 1837. Lyell had a clear perception of the regional diagenesis sequence in sedimentary rocks that remains valid today. It begins 'A' with deposition of unconsolidated sediment (protolith for future metamorphic rocks). As temperature and pressure increase with depth, a protolith passes through a diagenetic sequence from porous sedimentary rock through indurated rocks and phyllites 'A2' to metamorphic schists 'C1' in which the initial sedimentary components can still be discerned. Deeper still, the schists are reconstituted as gneiss 'C2' in which folia of residual minerals alternate with quartzo-feldspathic layers; partial melting continues as small batches of leucosome coalesce to form distinct layers in the neosome, and become recognizable migmatite 'D1'. The resulting leucosome layers in stromatic migmatites still retain water and gas in a discontinuous reaction series from the paleosome. This supercritical H2O and CO2 content renders the leucosome extremely mobile. Bowen described the process as being ‘In part due to … reactions between already crystallized mineral components of the rock and the remaining still-molten magma, and in part to reactions due to adjustments of equilibrium between the extreme end-stage, highly concentrated, "mother-liquor", which, by selective freezing, has been enriched with the more volatile gases usually termed "mineralizers," among which water figures prominently’. J.J. Sederholm (1926) described rocks of this type, demonstrably of mixed origin, as migmatites. He described the granitising 'ichors' as having properties intermediate between an aqueous solution and a very much diluted magma, with much of it in the gaseous state.
Partial melting, anatexis and the role of water The role of partial melting is demanded by experimental and field evidence. Rocks begin to partially melt when they reach a combination of sufficiently high temperatures (> 650 °C) and pressures (>34MPa). Some rocks have compositions that produce more melt than others at a given temperature, a rock property called fertility. Some minerals in a sequence will make more melt than others; some do not melt until a higher temperature is reached. If the temperature attained only just surpasses the solidus, the migmatite will contain a few small patches of melt scattered about in the most fertile rock. Holmquist 1916 called the process whereby metamorphic rocks are transformed into granulite ‘anatexis’. The segregation of melt during the prograde part of the metamorphic history (temperature > solidus) involves separating the melt fraction from the residuum, which higher specific gravity causes to accumulate at a lower level. The subsequent migration of anatectic melt flows down local pressure gradients with little or no crystallization. The network of channels through which the melt moved at this stage may be lost by compression of the melanosome, leaving isolated lenses of leucosome. The melt product gathers in an underlying channel where it becomes subject to differentiation. Conduction is the principal mechanism of heat transfer in the continental crust; where shallow layers have been exhumed or buried rapidly there is a corresponding inflection in the geothermal gradient. Cooling due to surface exposure is conducted very slowly to deeper rocks so the deeper crust is slow to heat up and slow to cool. Numerical models of crustal heating confirm slow cooling in the deep crust. Therefore, once formed, anatectic melt can exist in the middle and lower crust for a very long period of time. It is squeezed laterally to form sills, laccolithic and lopolithic structures of mobile granulite at depths of c. 10–20 km. In outcrop today only stages of this process arrested during its initial rapid uplift are visible. Wherever the resulting fractionated granulite rises steeply in the crust, water exits from its supercriticality phase, the granulite starts to crystallize, becomes firstly fractionated melt + crystals, then solid rock, whilst still at the conditions of temperature and pressure existing beyond 8 km. Water, carbon dioxide, sulphur dioxide and other elements are exsolved under great pressure from the melt as it exits from supercritical conditions. These components rise rapidly towards the surface and contribute to formation of mineral deposits, volcanoes, mud volcanoes, geysers and hot springs.
Color-banded migmatites
A leucosome is the lightest-colored part of migmatite. The melanosome is the darker part, and occurs between two leucosomes or, if remnants of the more or less unmodified parent rock (mesosome) are still present, it is arranged in rims around these remnants. When present, the mesosome is intermediate in color between leucosome and melanosome. The melanosome is a dark, mafic mineral band formed in migmatite which is melting into a eutaxitic texture; often, this leads to the formation of granite. The melanosomes form bands with leucosomes, and in that context may be described as schlieren (color banding) or migmatitic.
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